n-butanol detection is crucial to environmental protection and mankind’s health because of its toxicity and irritation to the respiratory system. Mono-metal oxide semiconductors (such as SnO2) often face disadvantages such as low sensitivity, insufficient selectivity, and a high detection limit. Constructing heterojunctions has been considered an efficient way to enhance the performance of MOS gas sensors. This work presents the synthesis of a series of β-CdSnO3-SnO2 nanorod-like heterostructures with good n-butanol sensitivity using a hydrothermal and post-calcination approach. The optimized CSO/SO-8 shows the highest response value about 90 to 100 ppm n-butanol at 190 °C, which is 15 times greater than that of SnO2, and 6.5 times more than that of β-CdSnO3. Meanwhile, the CSO/SO-8 also exhibits a short response and recovery time (1-2s/63s). More importantly, the detection limit is reduced to ppb level (50 ppb). The improved gas-sensitive properties can be explained by the formation of n-n heterojunctions between β-CdSnO3 and SnO2, which increases the content of chemisorbed oxygen, thus enhancing the performance of the gas-sensitive. Our work not only provides a material with n-n heterostructures but also the strategy for constructing heterojunctions to improve gas-sensitive properties.